Examining a diamond

ABSTRACT

In order to test whether a diamond has had a layer of synthetic diamond material deposited thereon, infrared radiation including radiation of wavelength substantially 7 μm to 25 μm preferably 7 μm to 10 μm emitted or transmitted by the diamond is observed, to detect differences between the compositions of different zones of the diamond.

BACKGROUND OF THE INVENTION

The present invention relates to a method of and apparatus for examininga diamond. For example, the invention may be used to test whether adiamond has had a layer of synthetic diamond deposited thereon. This isof particular importance in detecting whether the diamond comprises CVDdiamond material and also in locating such material if present.

The synthetic diamond material may be deposited on an uncut orpart-worked natural diamond which is then worked, for example, into around brilliant cut. Alternatively, the synthetic diamond coating may bedeposited onto a fully fashioned brilliant stone after working of thestone. The thickness of the synthetic diamond material layer may be verythin--it could be in the range from 1 μm to 0.1 mm.

The value of a diamond is in part dependent upon its weight.Accordingly, synthetic diamond material may be deposited onto naturalgem diamonds, before or after cutting of the diamond, to increase theweight of the finished product.

However, the value of a diamond also resides in its qualities ofauthenticity and uniqueness and in the fact that it is an entirelynatural product. Thus, a diamond that has not been enlarged bydeposition of synthetic diamond material has a value over a diamond thathas.

Over the years, a number of methods of synthesising diamond materialhave been developed. One of these methods is the chemical vapordeposition (CVD) technique, which is a low pressure technique involvingdeposition of synthetic diamond (referred to as CVD diamond material inthis specification) onto a substrate from a gas. CVD is the method whichis most likely to be used to deposit synthetic diamond onto a diamond.

A diamond artificially enlarged by deposition of CVD diamond material isreferred to in this specification as a "CVD/natural diamond doublet".

CVD diamond material may be deposited on a diamond substrate. The CVDdiamond material can replicate the structure of the diamond substrate(referred to "homoepitaxial growth"). The CVD/natural diamond doubletproduced can be identical in appearance, density and other commonphysical properties to an entirely natural stone and there may be aproblem in identifying such a CVD/natural diamond doublet.

It is an object of the present invention to provide a method of andapparatus for determining whether a diamond has had a layer of syntheticdiamond deposited thereon.

It is desired that the apparatus should be simple and inexpensive andmay be put into operation by a person with relatively little training.The method and apparatus should be capable of being operated reliablyand consistently by a practiced jeweller who has no training inlaboratory gemological analysis.

THE INVENTION

The present invention provides a method of testing whether a diamond hashad a layer of synthetic diamond deposited thereon, comprising comparingobservations of infra red radiation emanating from each of a pluralityof zones of the diamond, to detect differences between the compositionsof different zones of the diamond, the radiation observed includingradiation of wavelength substantially in the range 7 μm to 25 μm,preferably 7 μm to 10 μm.

The present invention also provides apparatus for testing whether adiamond has had a layer of synthetic diamond deposited thereon,comprising means for observing infra red radiation emanating from a zoneof the diamond which is substantially smaller than the total surfacearea of the diamond, the radiation observed including radiation ofwavelength substantially in the range 7 μm to 25 μm, preferably 7 μm to10 μm.

The present inventors have discovered that where the intensity of infrared radiation of the abovementioned waveband emanating from a diamond isdifferent between different zones, it may be concluded that the diamondhas had a layer of synthetic diamond deposited thereon. Where there areno such differences, it may be concluded that the diamond issubstantially of one type. Care should be taken, as explained below, todistinguish low observed intensities of radiation caused by internalreflection within the diamond from low observed intensities caused bydifferent compositions of diamond.

The present inventors have further discovered that the observations ofinfra red radiation may be compared with similar observations for adiamond of known type, to indicate whether the diamond under test is ofthe same type as the known diamond. If the known diamond is type IaAB,the diamond under test can be classified as definitely natural (typeIaAB) or not definitely natural, because type IaAB diamonds are alwaysnatural.

By "radiation emanating from a diamond" is meant radiation transmittedby the diamond, or emitted by the diamond, or both.

The present invention is based upon the observation that the majority ofnatural diamonds are type IaA or IaAB, whereas CVD diamond material isnormally type II, which transmits infra red radiation more strongly thantype IaAB and IaA diamond in the waveband 7 μm to 25 μm, the band ofinterest.

The present inventors have further discovered that observations of theinfra red radiation emitted from a diamond in the band of interest canbe used to detect a layer of synthetic diamond material deposited on anatural stone. It is simpler and more effective to observe emitted infrared radiation than transmitted radiation.

Preferably, a large number of zones of the diamond are observed, as CVDdiamond material, if present, may be localized on a part of the surfaceof the diamond. An image may of course be thought of as composed of aplurality of zones.

Preferably, enough zones to cover substantially the whole surface of thediamond are observed.

A preferred method of observing a large number of zones of the diamondat once comprises forming an image of the infra red radiation emanatingfrom the diamond. An image may of course be thought of as composed of aplurality of zones.

Preferably, the zones observed are small, being less than 1 mm,preferably less than 0.5 mm across. Where an image is formed, the sizeof the zones observed is only limited by the resolution of the imagingsystem.

An image of the diamond may be formed as viewed in a first directionrelative to the diamond, at least one further image of the diamond beingformed as viewed in a different direction relative to the diamond. Inthis way, zones not visible from the first direction may be examined.The normal to the interface of a synthetic diamond layer and naturalstone should be substantially normal to the optical axis of the imagingmeans, in order to be detected.

Preferably, an image of the diamond is formed using a long-wavelengththermal imaging camera, for example an Agema 900 Series thermal imagingcamera. Alternatively, an infra red imaging system having a cooled infrared filter passing radiation substantially in the waveband of interestmay be used.

Image processing means may be provided to improve contrast between areasof different emissivity. For example, areas of different temperaturerange may be presented in different colours. In order to distinguish thethermal emission of the diamond from that of the background, the imageof the diamond may be formed against a background at a differenttemperature to the diamond. A temperature difference in the range 5° to50° C. may be used, preferably in the range 20° to 30° C. Such atemperature difference will give adequate distinction between thediamond and the background, without the problems associated with greatertemperature differences.

Preferably, the diamond is at a higher temperature than the background,which may be achieved by heating the diamond or cooling the background,or both. However, the diamond could instead be cooled and/or thebackground heated.

The emission properties of the background in the waveband substantially7 μm to 25 μm may be tailored to enhance the view taken. The backgroundcould be made of a very good infra red emitter (for example, a mattblack material) or a very poor emitter (for example, a mirror-polishedsurface), instead of or in addition to being of a different temperature.

The diamond or background may be heated by being placed in contact witha heated support, by pre-heating it before placing it in the apparatus,by heating it in a warm air stream, or by using an infra red heatingsource or by a combination of these methods. If an infra red source isused, the source irradiation direction should not be coincident with theimaging direction, in order to avoid flooding the image of the diamondwith background radiation.

The background or the diamond may be cooled by being placed in contactwith a cool support, by pre-cooling or by directing cold air onto thesurface of the diamond or background or by a combination of thesemethods.

All parts of the diamond should be at substantially the sametemperature.

In order to assist the interpretation of the image of the diamond, thediamond may be irradiated with radiation which is substantiallytransmitted by all types of diamond, such as visible radiation, or whichis transmitted by no type of diamond, such as radiation of wavelengthless than 230 nm, so that a dark or light image of the entire diamond ora reference image may be formed. This reference image may then becompared to the image taken at the first mentioned wavelength,preferably with the diamond in the same configuration. The referenceimage can be used to distinguish features caused by different zoneshaving different emissivity from artifacts caused by internal reflectionof infra red radiation in the diamond. The reference image may be takenagainst a light background or against a dark background, and/or byirradiating the diamond with light in a different direction to thedirection of imaging.

The infra red radiation emitted by a zone of the diamond may be observedby using radiation collecting means, such as a light guide(advantageously a fiber-optic probe) and a radiation detector to whichthe collected radiation is directed. Any suitable known type ofapparatus for measuring the intensity of radiation emanating from thezone may be used.

If a fiber-optic probe is used, it may be fixed in means for mountingthe diamond, different zones of the diamond being placed in contact withthe end of the probe by manipulating the diamond. Any suitable knownmeans may be used for mounting the diamond, such-as a plinth or ledge.Alternatively, the diamond may be stationary and the fiber-optic probemanipulated around the diamond. When a light guide such as a fiber-opticprobe is used, a "map" or crude image of the emissivity at differentzones of the diamond may be produced by testing each such zone in turn.A crude image can be produced without expensive imaging apparatus. Theapparatus in this form may be suitable for automation.

A plurality of fiber-optic probes may be used for contacting differentzones of the diamond at the same time.

In order to avoid possible errors arising from poor or uncontrolledcontact of the fiber-optic probe with a zone of the diamond, a pluralityof observations at the same wavelength may be made for the same zone andcombined statistically to provide a statistically improved reading.

Optical means may be provided to allow a visible check to ensure thatthe end of the probe is in contact with the desired zone of the diamondbefore the intensity of radiation emitted is observed.

When the diamond is imaged, the image formed may be interpreted by theoperator by comparing the apparent intensity of emission of differentparts of the diamond or the intensities of radiation emitted bydifferent parts of the diamond may be measured and their differencescalculated. Assuming that the diamond is at a substantially uniformtemperature, parts of the diamond which have different infra redemissivity will appear to have different intensities of emission.Emissivity may be defined as the ratio of the power per unit arearadiated by the surface to that radiated by a black body at the sametemperature. Emissivity as a function of wavelength is defined as theratio of the power per unit area per unit wavelength interval radiatedby a surface at a given wavelength and temperature to that radiated by ablack body at the same wavelength and temperature.

Synthetic diamond material deposited on a natural stone will have alower emissivity than the neighboring natural diamond material and willappear to be cooler.

If a light guide is used to measure the intensity of radiation-emittedby a zone of the diamond, a signal dependent upon the emission intensitycan be read directly for each of the zones. A fiber-optic probe may beused, being moved over the diamond to look for parts which have anabnormally low emission intensity.

The radiation observed may comprise a narrow band of wavelengths lyingin the band of interest or a number of such narrow bands, but ispreferably a relatively broad band lying substantially in the band ofinterest, as this maximizes the amount of radiation observed. Theradiation studied may comprise radiation of wavelengths falling outsidethe above-mentioned range, for example as low as 6 μm or as high as 30μm. It is preferred that the power of radiation falling within the bandof interest will not be swamped out by radiation of wavelength fallingoutside this band.

It is strongly preferred that the radiation observed should includeradiation falling substantially in the range 7 μm to 10 μm. In thisrange, the absorption coefficient of natural diamond is significantlyhigher (being three or more times higher) than that of syntheticdiamond. Therefore, the contrast between the intensity of radiationemitted by natural and artificial diamond material is good. In the band10 μm to 25 μm, the intensity of radiation emitted by a natural diamondis detectably larger than that emitted by artificial diamond. However,the difference is small and if this band is observed, the observingmeans should be very sensitive to small differences in intensity ofradiation observed.

In a preferred embodiment, an emissivity difference of 0.5-5% or more,preferably 1% or more between different areas is interpreted to indicatethat diamond material of different compositions is present.

The diamond may be observed using image processing means such as acomputer. The image processing means may be set up to present the imageusing false colors to represent areas of different emission intensitybands. In this case, the threshold between the two emission intensitybands may be set such that areas of the diamond which are of differentcomposition but which are otherwise identical in orientation,temperature, surface finish etc fall into different color bands. Thiswill make visualization of areas of different composition easier.

The threshold may be set by the operator. For example, the operatorcould select an area of the image and program the image processing meansto assign a different color to an area of the diamond having anemissivity difference of 0.5-5% or more, preferably 1% or more, from it.

It is possible to irradiate the diamond with infra red radiation andobserve infra red radiation transmitted by different zones of thediamond, the observed radiation including radiation substantially ofwavelength substantially 7 μm to 25 μm.

If some zones of the diamond under test transmit less infra redradiation than others, it may be concluded that the diamond comprises asynthetic diamond layer.

Again, it is strongly preferred that the observed radiation shouldinclude radiation substantially of wavelength 7 μm to 10 μm.

The means for observing radiation transmitted by parts of the stone maybe the same as for the emission technique discussed above.

The diamond may be irradiated with infra red radiation by diffuselyirradiating one side of the diamond and viewing it from the other side,by placing the diamond in an integrating sphere irradiated with infrared radiation or by irradiation with a fiber-optic probe.

The radiation used to irradiate the diamond preferably falls only in theband of interest. However, it would be possible to obtain results evenif radiation falling outside the band of interest were also included.

The invention will be further described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a first embodiment of apparatusaccording to the invention;

FIG. 2 schematically shows an image of a CVD/natural diamond doubletproduced by the first embodiment of the invention; and

FIG. 3 is a schematic illustration of apparatus according to a secondaspect of the invention.

The apparatus shown in FIG. 1 comprises apparatus for observing theinfra red radiation emanating from a diamond, comprising imaging means 2in the form of a thermal camera set up to image radiation of wavelengthsfalling in the range 8-12 μm. A diamond 3 can be placed in theapparatus, mounted on conventional diamond mounting means such as a dop4 which allows the diamond to be manipulated and moved with respect tothe thermal imaging camera 2 to allow different views of the diamond 3to be taken.

The diamond 3 is viewed against a background 5. Means 6 and 7 may beprovided to create a temperature difference between the diamond and thebackground to allow the diamond to be more clearly imaged. The means 6comprise radiative heating means, for heating the diamond by infra redradiation. The means 6 emits radiation in a very broad band ofwavelengths.

The background 5 is cooled by a cooled liquid pumped through thebackground by apparatus 7.

In order to provide a reference image of the diamond 3 to allow parts ofthe diamond 3 which are of interest to be more precisely located andinterpreted, a lamp 8 is provided to illuminate the diamond 3 withvisible radiation. The camera 2 may be such that it can image visibleradiation, or an extra camera may be required.

The apparatus is contained in a light tight box 9 to prevent stray infrared radiation entering the imaging system 2.

The imaging system 2 produces an output on monitor 10 which can bestudied by the operator.

FIG. 2 shows a typical image produced on the monitor 10. A CVD/naturaldiamond doublet mounted on a dop 4 is shown. Natural parts 11 of thestone appear relatively bright, CVD diamond material parts 12 appearingcomparatively dark. The image will show a complex pattern of facets. Areference image may be produced using visible light from lamp 8, todistinguish such facets from features of different emissivity.

FIG. 3 shows an alternative apparatus according to the invention. Theapparatus comprises diamond mounting means 13 such as a dop for mountinga diamond 3. A fiber-optic probe 14 is provided which may be contactedwith the surface of the diamond 3 at a number of positions.

The fiber-optic probe 14 passes infra red radiation to a detector 15which produces a reading or signal dependent upon the intensity ofradiation emitted by the part of the stone in contact with the end ofthe fiber-optic probe 14. The fiber-optic probe 14 and diamond may becontained in a light tight box 16 to prevent stray infra-red radiationentering the detector 15.

The present invention has been described above purely by way of example,and modifications can be made within the invention. The invention alsoconsists in any individual features described or implicit herein, orshown or implicit in the drawings or any combination of such features orany generalization of any such features or combinations.

We claim:
 1. A method of testing whether a diamond has had a layer ofsynthetic diamond deposited thereon, comprising comparing observationsof infra red radiation emanating from each of a plurality of zones ofthe diamond, to detect differences between the compositions of differentzones of the diamond, the infra red radiation including radiation ofwavelength substantially 7 μm to 25 μm.
 2. The method of claim 1,wherein the infra red radiation emitted by said zones is observed. 3.The method of claim 2, wherein the infra red radiation emitted by saidzones is observed by forming an image of the diamond.
 4. The method ofclaim 3, wherein the image of the diamond is formed against a backgroundwhich is at a different temperature to the diamond.
 5. The method ofclaim 4, wherein the diamond is at a higher temperature than thebackground.
 6. The method of claim 4 or 5, wherein the diamond isheated.
 7. The method of claim 4 or 5, wherein the background is cooled.8. The method of any of claims 2 to 5, wherein an image of the diamondis formed as viewed in a first direction relative to the diamond, atleast one further image of the diamond being formed as viewed in adifferent direction relative to the diamond.
 9. The method of any ofclaims 1 to 5, further comprising irradiating a plurality of differentzones of the diamond with reference radiation which is substantiallytransmitted by all types of diamond or substantially absorbed by alltypes of diamond, and comparing observations of the reference radiationtransmitted or reflected by the diamond with observations of the firstmentioned radiation emanating from the diamond.
 10. The method of claim9, wherein an image of the reference radiation transmitted or reflectedby the diamond is formed.
 11. The method of claim 9, wherein thereference radiation comprises visible radiation.
 12. The method of anyof claim 1 to 5, further comprising the step of observing infra redradiation including radiation of wavelength substantially 7 μm to 25 μmemanating from zones of a diamond of known type and comparingobservations of the known diamond and the diamond under test, todetermine whether the diamond under test is of the same type as theknown diamond.
 13. The method of any of claims 1 to 5, furthercomprising the step of identifying the diamond as a CVD/natural diamonddoublet, if the intensity of radiation emanating from some zones of thediamond is different to the radiation emanating from other zones. 14.The method of any of claims 1 to 5, wherein the radiation observedincludes radiation of wavelength falling in the range substantially 7 μmto 10 μm.
 15. The method of claim 1, wherein the radiation emanatingfrom said zones is observed using radiation collecting means and aradiation detector.
 16. The method of claim 15, wherein the diamond isplaced in an integrating enclosure, the luminous flux intensity of thefirst mentioned radiation in the enclosure being detected by thedetector.
 17. The method of claim 15, wherein infra red radiationemitted by said zones is collected by a light guide and deliveredthereby to the detector.
 18. The method of claim 1, comprisingirradiating the diamond with infra red radiation including radiation ofwavelength substantially 7 μm to 25 μm, and observing radiationsubstantially of wavelength substantially 7 μm to 25 μm transmitted bydifferent zones of the diamond.
 19. The method of claim 18, wherein theradiation transmitted by said zones is observed by forming an image ofthe diamond.
 20. Apparatus for testing whether a diamond has had a layerof synthetic diamond deposited thereon, comprising means for observinginfra red radiation emanating from a zone of the diamond which issubstantially smaller than the total surface area of the diamond, theinfra red radiation including radiation of wavelength substantially 7 μmto 25 μm and means for forming an image of an area of the diamondincluding said zone.
 21. The apparatus of claim 20, wherein the infrared radiation observed includes radiation of wavelength substantially 7μm to 10 μm.
 22. The apparatus of claim 20 to 21, further comprisingmounting means for alterably mounting the position of the diamond withrespect to the observing means.
 23. The apparatus of claim 20, furthercomprising means for defining a background against which the diamond isimaged.
 24. The apparatus of claim 23, further comprising means forcreating a temperature difference between the diamond and thebackground.
 25. The apparatus of claim 24, wherein the temperaturedifference creating means is for heating the diamond.
 26. The apparatusof any one of claims 23 to 25, wherein the infra red radiation observedincludes radiation of wavelength substantially 7 μm to 10 μm.
 27. Theapparatus of claim 24, wherein the temperature difference creating meansis for cooling the background.
 28. The apparatus of claim 20, whereinthe means for observing the emitted infra red radiation comprises aradiation collector and a detector.
 29. The apparatus of claim 28,wherein the radiation collector comprises a light guide for collectinginfra red radiation emitted by said zone of the diamond and fordelivering radiation thus collected to the detector.
 30. The apparatusof any of claims 20, 21, 28, or 29, further comprising means forirradiating the diamond with reference radiation which is substantiallytransmitted by all types of diamond or substantially absorbed by alltypes of diamond, and means for observing reference radiationtransmitted or reflected by the diamond.
 31. The apparatus of claim 30,wherein an image of the transmitted or reflected reference radiation isformed.
 32. The apparatus of claim 30, wherein the reference radiationcomprises visible radiation.
 33. The apparatus of claim 20, furthercomprising means for irradiating the diamond with infra red radiationincluding radiation of wavelength substantially 7 μm to 25 μm, theobserving means being for observing radiation substantially ofwavelength substantially 7 μm to 25 μm transmitted by said zone of thediamond.
 34. The apparatus of claim 33, wherein the irradiating means isfor irradiating the diamond with infra red radiation including radiationof wavelength substantially 7 μm to 10 μm and the observing means is forobserving radiation of wavelength substantially 7 μm to 10 μm.
 35. Theapparatus of any of claims 20, 21, 28, 29, 33, or 34, wherein the meansfor observing infra red radiation is configured to examine a zone of thediamond which zone is substantially 0.5 mm across.
 36. Apparatus fortesting whether a diamond has had a layer of synthetic diamond depositedthereon, comprising means for observing infra red radiation emanatingfrom a zone of the diamond which is substantially smaller than the totalsurface area of the diamond, the infra red radiation including radiationof wavelength substantially 7 μm to 25 μm, and mounting means foralterably mounting the position of the diamond with respect to theobserving means.
 37. Apparatus for testing whether a diamond has had alayer of synthetic diamond deposited thereon, comprising a radiationcollector for collecting infra red radiation emanating from a zone ofthe diamond which is substantially smaller than the total surface areaof the diamond, and a radiation detector for detecting the collectedradiation of wavelength substantially 7 μm to 25 μm.
 38. The apparatusof claim 37, wherein the radiation collector comprises a light guide forcollecting infra red radiation emitted by said zone of the diamond andfor delivering radiation thus collected to the detector.
 39. Apparatusfor testing whether a diamond has had a layer of synthetic diamonddeposited thereon, comprising means for observing infra red radiationemanating from a zone of the diamond which is substantially smaller thanthe total surface area of the diamond, the infra red radiation includingradiation of wavelength substantially 7 μm to 25 μm, means forirradiating the diamond with reference radiation which is substantiallytransmitted by all types of diamond or substantially absorbed by alltypes of diamond, and means for observing reference radiationtransmitted or reflected by the diamond.
 40. The apparatus of claim 39including means for forming an image of the transmitted or reflectedreference radiation.
 41. The apparatus of claim 40, wherein thereference radiation comprises visible radiation.
 42. The apparatus ofclaim 39 wherein the reference radiation comprises visible radiation.43. Apparatus for testing whether a diamond has had a layer of syntheticdiamond deposited thereon, comprising means for observing infra redradiation emanating from a zone of the diamond which is substantiallysmaller than the total surface area of the diamond, the infra redradiation including radiation of wavelength substantially 7 μm to 25 μm,and means for irradiating the diamond with infra red radiation includingradiation of wavelength substantially 7 μm to 25 μm, the observing meansbeing for observing radiation substantially of wavelength substantially7 μm to 25 μm transmitted by said zone of the diamond.
 44. The apparatusof claim 43 wherein the irradiating means is for irradiating the diamondwith infra red radiation including radiation of wavelength substantially7 μm to 10 μm and the observing means is for observing radiation ofwavelength substantially 7 μm to 10 μm.
 45. The apparatus of any one ofclaims 37 to 44, wherein the infra red radiation observed includesradiation of wavelength substantially 7 μm to 10 μm.
 46. Apparatus fortesting whether a diamond has had a layer of synthetic diamond depositedthereon, comprising support means for supporting a diamond, and meansfor observing infra red radiation emanating from a zone of a diamondsupported by the support means which zone is substantially smaller thanthe total surface area of the diamond, the infra red radiation includingradiation of wavelength substantially 7 μm to 25 μm, and wherein theinfra red radiation observed includes radiation of wavelengthsubstantially 7 μm to 10 μm.
 47. The apparatus of claim 46 and furtherincluding a substantially light-light enclosure surrounding the supportmeans.